Clean up renderer code

Remove code that is no longer relevant after the removal of ncurses
renderer. This commit also fixes background color issue on tcell-based
FullscreenRenderer (Windows).

src/tui/dummy.go

@@ -33,7 +33,6 @@ func (r *FullscreenRenderer) Refresh()    {}
 func (r *FullscreenRenderer) Close()      {}
 
 func (r *FullscreenRenderer) DoesAutoWrap() bool { return false }
-func (r *FullscreenRenderer) IsOptimized() bool  { return false }
 func (r *FullscreenRenderer) GetChar() Event     { return Event{} }
 func (r *FullscreenRenderer) MaxX() int          { return 0 }
 func (r *FullscreenRenderer) MaxY() int          { return 0 }